1. Field of the Invention
The present invention relates to an apparatus for detecting the surface potential of an object in a non-contact manner, and more particularly, it relates to a surface potential detector which can reduce output variation caused by variation in the set distance between a measured surface and the detector.
2. Description of the Prior Art
A chopper type or a vibrating reed type detector is well known in the art as an apparatus for detecting the potential of a surface to be measured in a non-contact manner. In either type of detector, an amount of lines of electric force formed between a measured surface and detecting electrodes are changed to detect the potential of the measured surface on the basis of variation in charges induced on the detecting electrodes. When the distance between the measured surface and the detector is varied, therefore, the amount of the lines of electric force is varied whereby the output of the detector is inevitably changed.
Thus, various correction means have been proposed in the art in order to correct such output change based on the variation in distance between the measured surface and the detector.
FIG. 20 shows the structure of a conventional surface potential detector which is disclosed in Japanese Patent Laying-Open Gazette No. 152959/1985. Referring to FIG. 20, the conventional surface potential detector is provided with first and second detecting electrodes 6 and 7 which are different in distance from a measured surface 1 to obtain output difference between the first and second detecting electrodes 6 and 7, thereby to correct the measuring output on the basis of the output difference.
As shown in FIG. 20, a window 8a of a prescribed configuration is formed in a case 8 in order to guide lines X of electric force from the measured surface 1. Namely, the window 8a is provided as a single part while the detecting electrodes 6 and 7 are provided in plural, and hence the lines X of electric force reach the detecting electrodes 6 and 7 through the same window 8a, as typically shown in FIG. 21.
Such structure is also applicable to a vibrating reed type surface potential detector in such a manner that, as typically shown in FIG. 22, first and second detecting electrodes 11 and 12 are provided to be different in distance from a measured surface 1, so that the detector output can be corrected on the basis of output difference between the detecting electrodes 11 and 12.
In such a surface potential detector provided with the detecting electrodes 6 and 7 or 11 and 12 different in distance from the measured surface 1, however, the output difference between the two detecting electrodes is the function of the distance in practice. Therefore, it is not possible to obtain correct output with completely corrected distance by the correction method utilizing the output difference between two detecting electrodes. In other words, the output difference between the detecting electrodes 11 and 12 is not constant, as shown in FIG. 23.